HospitalResident: ValueError when using optimal="hospital"

[costructuralist]

Hi there! Thanks for this great library.

I'd like to know whether the following is expected behaviour. Using the HospitalResident algorithm, I have the scenario below. All hospitals rank all residents. The hospitals do not have enough capacity, though: there are 8 residents and only 6 spots.

When using the optimal="resident" option, the algorithm succeeds and says that residents S3 and S1 were not placed — which is expected.

Using the optimal="hospital" option, however, an exception is raised:

 File "<...snip...>\Lib\site-packages\matching\algorithms\hospital_resident.py", line 141, in hospital_optimal
    successors = resident.get_successors()
                 ^^^^^^^^^^^^^^^^^^^^^^^^^
 File "<...snip...>\Lib\site-packages\matching\players\player.py", line 52, in get_successors
    idx = self.prefs.index(self.matching)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: H2 is not in list

The code is as follows:

resident_preferences = {
    "S1": ["H1", "H2"],
    "S2": ["H3", "H1"],
    "S3": ["H1", "H3"],
    "S4": ["H3", "H2"],
    "S5": ["H1", "H3"],
    "S6": ["H2", "H1"],
    "S7": ["H3", "H1"],
    "S8": ["H2", "H1"],
}

hospital_preferences = {
    "H1": ["S5", "S6", "S3", "S4", "S1", "S2", "S7", "S8"],
    "H2": ["S8", "S2", "S3", "S4", "S5", "S6", "S7", "S1"],
    "H3": ["S7", "S2", "S3", "S4", "S5", "S6", "S1", "S8"],
}

hospital_capacities = {
    "H1": 2,
    "H2": 2,
    "H3": 2,
}

game = HospitalResident.create_from_dictionaries(
    resident_preferences, hospital_preferences, hospital_capacities
)

# solution = game.solve(optimal="resident") <-- this works
solution = game.solve(optimal="hospital") # <-- this doesn't